In recent years, higher integration of semiconductor devices places demand for higher flatness of semiconductor wafers used in such devices. The higher flatness up to near the edge of wafers is also needed to raise the yield of semiconductor chips.
The shape of finished semiconductor wafers depends on a final mirror polishing process. For a 300-mm-diameter silicon wafer, in particular, primary polishing by double-side polishing is performed to meet strict specifications of the flatness, and then secondary polishing and finish polishing by single-side polishing are performed to remove scratches on a wafer surface or to improve surface roughness.
The secondary polishing and finish polishing by single-side polishing need to maintain or improve flatness obtained by the primary polishing by double-side polishing and to finish the front surface of a wafer into a perfect mirror-surface with no defects, such as scratches.
As shown in FIG. 13, for example, a typical single-side polishing apparatus includes a turn, table 103 to which a polishing pad 102 is attached, a polishing-agent-supply mechanism 104, a polishing head 101. Such a polishing apparatus 110 holds a workpiece W with the polishing head 101, supplies a polishing agent 105 from the polishing-agent-supply mechanism 104 onto the polishing pad 102, rotates both the turn table 103 and the polishing head 101, and polishes the workpiece W by bringing a front surface of the workpiece into sliding contact with the polishing pad 102.
One exemplary method of holding a workpiece with a polishing head is to attach the workpiece to a flat workpiece-holding board through an adhesive such as wax. As shown in FIG. 14, there is also a polishing head 121 of a waxless type that holds a workpiece W with a commercially available template assembly 113 adhering to a workpiece-holding board 112. The template assembly has a template 113b, for preventing the workpiece from coming off, attached to an elastic film 113a called a backing film.
Other exemplary polishing heads of a waxless type currently used include a polishing head 131, as shown in FIG. 15, that includes a backing film 113a attached to a surface of a workpiece-holding board 112, instead of the commercially available template, and an annular guide ring 113b, for preventing a workpiece from coming off, provided around the side surface of the workpiece-holding board.
Although the workpiece holding board 112 is commonly composed of a highly-flat ceramic plate, this polishing head has a problem in that variation in thickness of the backing film 113a, etc., produces a small pressure distribution, resulting in waviness of the polished front surface of the workpiece and hence there is a problem that deteriorates the flatness of the workpiece.
Accordingly, there is also disclosed the so-called rubber chuck type of polishing head having a rubber film for holding a workpiece in place of the workpiece-holding board, in which the workpiece is pressed against a polishing pad by supplying pressurized fluid, such as air, to the back surface of the rubber film to inflate the rubber film with a uniform pressure (see Patent Document 1, for example).
FIG. 16 schematically illustrates an example of a configuration of the rubber chuck type of polishing head. The essential parts of the polishing head 141 are an annular rigid ring 144 made of e.g., stainless steel, a rubber film 143 attached to the rigid ring 144, and a back plate 145 connected to the rigid ring 144. The rigid ring 144, the rubber film 143, and the back plate 145 define a sealed space 146. On a lower surface portion of the rubber film 143, a backing film 148 is attached, and an annular template 147 is provided concentrically with the rigid ring 144. Pressurized fluid is supplied through the center of the back plate 145 with a pressure-adjusting mechanism 150 to adjust the pressure in the sealed space 146. The back plate 145 is connected to a polishing head body 149 having a pressing means (not shown) for pressing the back plate 145 toward a polishing pad.